module Day14

open System.IO
open FParsec
open Common

let sandSpawnPos = Vec2(500, 0)

let sandMoveOffsets =
    [ Vec2.down
      Vec2.downLeft
      Vec2.downRight ]

let buildCaveScan =
    let parsePath =
        let py = pint32 |>> (~-) // mirror Y coordinate
        let ppoint = pint32 .>> (pchar ',') .>>. py |>> Vec2
        let ppath = sepBy ppoint (pstring " -> ")
        Util.parse ppath

    let pathToPositions =
        Seq.pairwise
        >> Seq.collect Vec2.lineBetween
        >> Set

    Seq.map (parsePath >> pathToPositions)
    >> Seq.reduce (+)

let solution1 input =
    let initialCaveScan = buildCaveScan input
    let voidY = initialCaveScan |> Seq.map Vec2.getY |> Seq.min

    let settleNewUnit caveScan =
        let rec fall pos =
            if Vec2.getY pos <= voidY then
                None
            else
                sandMoveOffsets
                |> Seq.map ((+) pos)
                |> Seq.tryFind (Util.notIn caveScan)
                |> function
                    | Some (nextPos) -> fall nextPos
                    | None -> Some(pos)

        caveScan
        |> match fall sandSpawnPos with
           | Some (settledPos) -> Set.add settledPos
           | None -> id

    initialCaveScan
    |> Seq.unfold (fun caveScan -> Some(caveScan, settleNewUnit caveScan))
    |> Seq.pairwise
    |> Seq.takeWhile (fun (a, b) -> a <> b)
    |> Seq.length

let solution2 input =
    let caveScan = buildCaveScan input
    let floorY = caveScan |> Seq.map Vec2.getY |> Seq.min |> (+) -2

    let neighbours pos =
        sandMoveOffsets
        |> List.map ((+) pos)
        |> List.filter (fun pos -> Util.notIn caveScan pos && Vec2.getY pos <> floorY)

    let rec dfs vis =
        function
        | h :: t -> dfs (Set.add h vis) (List.filter (Util.notIn vis) (neighbours h) @ t)
        | [] -> Set.count vis

    dfs Set.empty [ sandSpawnPos ]

let test = File.ReadLines("test.txt")
assert (solution1 test = 24)
assert (solution2 test = 93)

let input = File.ReadLines("input.txt")
printfn "%d" <| solution1 input
printfn "%d" <| solution2 input